Tissue engineering is a promising method for the regeneration of degenerated or lost cartilage. This approach generally involves the use of cells placed in three-dimensional scaffolds, the latter acting as a temporary artificial extracellular matrix (ECM). Injectable hydrogels may serve as temporary scaffolds to guide cell attachment and differentiation of chondrocytes and/or their progenitor cells, resulting in newly formed cartilage tissue. Compared to preformed hydrogels, injectable hydrogels have various advantages. They can be applied via a minimally invasive surgical procedure. They can fill irregular-shaped defects and allow easy incorporation of cells and bioactive molecules.
Therefore, in recent years injectable hydrogels have received much attention in cartilage tissue engineering. Several chemical crosslinking methods, such as photopolymerization, Schiff-base formation, and Michael-type addition reactions, have been employed to obtain injectable hydrogels that gel in situ. Recently, an efficient method, i.e. horseradish peroxidase (HRP)-mediated chemical crosslinking, has been developed to produce injectable hydrogels. Using this approach, Lee et al. reported on hyaluronic acid-based injectable hydrogels for protein release and Sakai et al. prepared gelatin-based injectable hydrogels in vitro and indicated their potential application in tissue engineering in vivo (Lee et al. Journal of Controlled Release, 2009, 134, 186-193; Sakai et al. Biomaterials, 2009, 30, 3371-3377). In Kurisawa et al. (Chem. Commun., 2005, 4312-4314) injectable biodegradable hydrogels composed of hyaluronic acid-tyramine conjugates for drug delivery and tissue engeneering are described. In Jin et at (Biomaterials 28 (2007), 2791-2800) hydrogels formed by enzymatic crosslinking of dextran-tyramine conjugates are described.
We previously showed that fast in situ forming injectable hydrogels can be obtained via enzymatic crosslinking of dextran-tyramine conjugates (Dex-TA) or chitosan-phloretic acid conjugates in the presence of HRP and hydrogen peroxide (Jin R. et al., Biomaterials. 2009 (13):2544-51 and Jin R. et al., Biomaterials, 2007 (18):2791-800.)
These hydrogels had good mechanical properties and low cytotoxicity. Chondrocytes incorporated in the gels remained viable and were capable of maintaining their phenotype and producing cartilaginous tissue. However, a disadvantage is that these gels have a low storage modulus. Increasing the storage modulus by increasing the polymer concentration results in hydrogels wherein chondrocytes change their chondrocytic phenotype rapidly. Therefore, it is an objective of the invention to provide a novel polymer suitable for forming biocompatible hydrogels, wherein chondrocytes maintain their phenotype. Furthermore, it is an objective of the invention to provide conjugates which can be used to prepare hydrogels which display better mechanical properties and/or are better broken down in a living animal.